fontFeatures: Python library for manipulating OpenType font features

Contents:

OpenType fonts are “programmed” using features, which are normally authored in Adobe’s feature file format. This like source code to a computer program: it’s a user-friendly, but computer-unfriendly, way to represent the features.

Inside a font, the features are compiled in an efficient internal format. This is like the binary of a computer program: computers can use it, but they can’t do else anything with it, and people can’t read it.

The purpose of this library is to provide a middle ground for representing features in a machine-manipulable format, kind of like the abstract syntax tree of a computer programmer. This is so that:

  • features can be represented in a structured human-readable and machine-readable way, analogous to the XML files of the Unified Font Object format.
  • features can be more directly authored by programs (such as font editors), rather than them having to output AFDKO feature file format.
  • features can be easily manipulated by programs - for example, features from two files merged together, or lookups moved between languages.
How is this different from fontTool’s feaLib? I’m glad you asked. feaLib translates between the Adobe feature file format and a abstract syntax tree representing elements of the feature file - not representing the feature data. The AST is still “source equivalent”. For example, when you code an aalt feature in feature file format, you might include code like feature salt to include lookups from another feature. But what’s actually meant by that is a set of lookups. fontFeatures allows you to manipulate meaning, not description.

Components

fontFeatures consists of the following components:

  • fontFeatures itself, which is an abstract representation of the different layout operations inside a font.
  • fontFeatures.feaLib (included as a mixin) which translates between Adobe feature syntax and fontFeatures representation.
  • fontFeatures.ttLib which translates between OpenType binary fonts and fontFeatures representation. (Currently only OTF -> fontFeatures is partially implemented; there is no fontFeatures -> OTF compiler yet.)
  • fontFeatures.fontDameLib which translate FontDame text files into fontFeatures objects.

And the following utilities:

  • otf2fea: translates an OTF file into Adobe features syntax.
  • txt2fea: translates a FontDame txt file into Adobe features syntax.

Indices and tables